The tube-bundle heat exchangers described in industry are used for heating or cooling fluids and for vaporizing volatile substances from viscous media. They usually consist of tubes through which the medium to be devolatilized or to be heated flows and around the outside of which a heating medium flows. The apparatuses are referred to, for example, as flat-tube heat exchangers, falling-tube evaporators or flat-tube evaporators. Fields of use of tube-bundle heat exchangers are, for example, evaporators for polymer solutions, air conditioning systems or refrigerators.
EP 0772017 A1 describes a heat exchanger tube for use in tube-bundle heat exchangers, preferably in condensing boilers for cooling heating gases. The tube has an approximately circular cross section at its ends and an oval or rectangular cross section in the tube part located in between, which cross section may be constant or variable. The inside of the tube is provided with beads, grooves or indentations in order to generate a turbulent boundary layer and to improve the heat transfer. However, the generation of turbulent flow with the aid of indentations, etc. does not work in the case of the highly viscous polymer melts exhibiting laminar flow.
The tube-bundle heat exchanger apparatus described in EP 0218104 A1 operates in the low-pressure range according to the cross counterflow principle and is used for gas/gas and liquid systems, in particular for combustion processes with large amounts of gas and high waste gas temperatures and for processes in which a solvent circulates. The apparatus consists of tubes which have a hexagonal cross-sectional profile at their ends and an elliptical or round cross section in the middle part (about 90% of the tube length). The advantage of the hexagonal profile is that the tube ends can be joined directly to one another and a tight cavity thus forms between the tubes without preperforated end trays being required. However, this design is disadvantageous when the side walls of the heat exchanger jacket are connected to the joined tubes. It is necessary to weld on prepunched metal sheets which simulate the contour of the tubes joined to one another.
The heat exchanger described in DE 3212727 A1 (corresponding to U.S. Pat. No. 4,667,734) and intended for cooling or heating highly viscous, in particular structurally viscous, flowable substances consists of rows of parallel tubes which are arranged offset by 90° in planes parallel to one another. The heating or cooling medium flows in the tubes, and the fluid to be heated or to be cooled flows on their outside, perpendicular to the plane of the rows of tubes. The tubes are in the form of round or flat tubes. A disadvantage of this arrangement is that many “dead spaces” exist between the intersecting tubes, in which dead spaces the medium to be heated or to be cooled collects and undergoes thermal decomposition. Moreover, this design is very complicated and difficult to scale up for larger production plants.
A brochure of Schrader Verfahrenstechnik GmbH discloses a flat-tube heat exchanger. This flat-tube heat exchanger is used for condensing vapors or as a heating element in concentration plants. The apparatus described is a tube-bundle heat exchanger which consists of a heat exchanger jacket in which flat tubes arranged in rows are present.
Improved heat transfer and less fouling in comparison with round tubes are mentioned as advantages.
An application for polymers or polymer melts is not known.
Heat exchanger tubes having flat, oval or elliptical cross section and the production thereof are described in the prior art. DE 805 785 A1, for example, discloses a plate-type heating element which has a largely rectangular cross section. The plate-type heating element is however produced by bending over the two sides of a metal sheet and welding in further metal strips and stiffening profiles. This apparatus is taken as a basis in DE 2 506 434 A1 (corresponding to U.S. Pat. No. 4,024,619), the heat exchanger tube being produced from a single metal strip by bending over, and a supporting flange being integrated. The overlapping ends are then welded. A similar flat tube is described in DE 19 857 510 (corresponding to U.S. Pat. No. 6,129,147), but here the supporting flange consists of an edge of the metal strip which is bent from the surface of the tube to the tube interior and back again so that two chambers form. The publication DE 7 837 359 describes a flat heat exchanger tube which, in contrast to the abovementioned one, ensures strong and liquid-tight connection in the overlap region over the entire period of use of the tube.
A disadvantage of all designs mentioned is the complicated production of the tubes, at least one shaping and one joining process being necessary. It is an object of the invention to use a compact tube in which only one shaping process is necessary. Moreover, it is intended to dispense with the joint seam, which can lead to disadvantageous changes in the melt flow.
It is therefore the object of the invention to remove volatile substances from highly viscous polymer solutions or melts under particularly gentle conditions, i.e. with as low a thermal load as possible (temperature and residence time) and narrow residence time distribution, and to provide a suitable apparatus which does not have the disadvantages of the known heat exchangers.